1. Field of the Invention
The invention relates to telecommunication equipment, and in particular, to a method for echo cancellation and suppression.
2. Background Art
Echo Cancellers
Echo Canceller (EC) is a device or software which could cancel echo signals by using some reference signal. The reference signal is also called received signal (Rx signal). The echo signal is also called echo return signal which is mixed in the transmission signal (Tx signal). There are two major types of echo cancellers: one is called Acoustic Echo Canceller (AEC); another one is named as Line Echo Canceller (LEC). Obviously, AEC is used to cancel acoustic echoes and LEC is employed to cancel line echoes.
Line echoes result from imperfect impedance matching by hybrids in two-to-four wire signal conversion. Acoustic echoes happen in the field of telecommunication equipment where acoustically coupled echoes are transmitted back to the receiving party while also providing a full duplex connection; it is also a problem in the filed of audio conferencing to prevent adding noise of the inactive talkers into the output while also providing a conference bridge where two or more conferees can talk at one. Some echo is acceptable in voice conversations, however, users are annoyed by listening to their own speech delayed by the round-trip time of the system.
Echo canceller often consists of an adaptive filter with Least Mean Square (LMS) algorithm which generates an echo replica signal similar to the echo signal. A subtraction between the echo return signal and the echo replica signal is conducted in order to cancel the echo return signal. Due to the fact that the replica signal could not be perfect enough to duplicate the echo return signal for various reasons, there is always some residual echo left in the transmission signal. Echo Suppression is an apparatus which could perform echo reduction and/or echo elimination, in particular for reducing or eliminating residual echo signals left after echo canceller. Due to the inherent problems associated with echo cancellers, many solutions rely significantly on an additional echo suppression stage.
Echo Suppression
Echo Suppressor is a device or software which could dramatically reduce the (residual) echo energy without significantly distorting the non-echo speech signal. Although Echo Suppressor can work alone without combining with echo canceller, it often works as a compensation of echo canceller. Not only Echo Suppressor can dramatically suppress the residual echo energy but also reduce background noise energy by taking the advantage of available parameters. Echo Suppressor could be viewed as an independent function or just a part of Echo Canceller system.
Existing Echo Suppression methods are mostly based on the following approaches:                Spectrum Echo Suppression is one of the popular approaches. This approach requires FFT operation to transform time domain signals into frequency domain; so, the complexity is not low. This approach needs to carefully tune all the parameters in order to avoid possible “music noise” before doing Inverse FFT of the modified spectrum back into time domain.        Another popular way is called NLP (Non-Linear Processor) which replaces the residual echo signal with random noise or turns off transmission in the reverse direction while one person is talking and double talk does not exist. This method is simple but requires very robust double talk detection and residual echo detection. Due to the fact that it is very difficult to have a precise double talk detector or perfect residual echo detection, this approach often introduces “choppiness”, especially in noisy environments.        
This invention proposed an Echo Suppression approach which can achieve robust performance with low complexity level.